Borrego Solar: Plymouth Public Schools

The Plymouth Public Schools (PPS) district is one of the first in the country to purchase 100% of its energy from solar sources. Initially, Borrego Solar planned to install 8–10 PV arrays on campuses across the PPS district. A combination of older roof structures and snow loads drove PPS and Borrego Solar to an alternative plan to develop two expansive ground-mounted solar arrays on nearby private property. Together, the arrays cover more than 23 acres of land—an area roughly the size of 18 professional football fields. The larger array comprises 23,647 solar modules on land leased from Plympton Sand & Gravel.

In Massachusetts, large PV installations can have separate entities as the host (land owner), energy off-taker (end user) and system owner (investor). In the Plympton project, the host generates revenue by leasing the land used to house the project. As the off-taker, PPS agrees to purchase the solar power from the owner at a predetermined rate that is $0.06/kWh less than what it would be from NSTAR Electric and Gas. With a power purchase agreement, PPS will save approximately $400,000 on energy annually ($8.5 million over the 20-year PPA term), and the district avoided incurring the up-front project costs of about $11 million.

“The PPS solar installation in Plympton, Massachusetts, the largest of the district’s arrays, won a Project of Distinction award at this year’s PV America East conference in Boston. Thanks to innovative net metering in the state, PPS is able to virtually apply the solar energy produced off-site to its own utility meters.”

Philip Hall, director of marketing, Borrego Solar

Overview

DESIGNER: Gabe Landes, senior design engineer, Borrego Solar, borregosolar.com

INSTALLERS: Charles Barbanti, project manager; Joe Daugirda, site superintendent, Borrego Solar

DATE COMMISSIONED: October 2013

INSTALLATION TIME FRAME: 150 days

LOCATION: Plympton, MA, 42°N

SOLAR RESOURCE: 3.81 kWh/m2/day

ASHRAE DESIGN TEMPERATURES:  88°F 2% avg. high, 0°F extreme min.

ARRAY CAPACITY: 5.597 MWdc

ANNUAL AC PRODUCTION: 7,235 MWh

Equipment Specifications

MODULES: 8,502 Sharp ND-235QCJ, 235 W STC, +5/-0%, 8.02 Imp, 29.3 Vmp, 8.60 Isc, 37.2 Voc; 8,125 Sharp ND-240QCJ, 240 W STC, +5/-0%, 8.19 Imp, 29.3 Vmp, 8.75 Isc, 37.5 Voc; 6,721 Yingli YL235P-29b, 235 W STC, +5/-0%, 7.97 Imp, 29.5 Vmp, 8.54 Isc, 37 Voc; 299 Inventec Energy IECS-6P6A-235, 235 W STC, 7.92 Imp, 29.69 Vmp, 8.55 Isc, 37.03 Voc

INVERTERS: Eight SMA Sunny Central 500HE-US, 500 kW, 600 Vdc maximum input, 330–600 Vdc MPPT range, 200 Vac nominal output, two inverters per 1,000 kVA step-up transformer

ARRAY: 13 modules per source circuit, 26 source circuits per combiner (typical), nine combiners per inverter; 1,819 source circuits total, 5.597 MWdc array capacity total

ARRAY INSTALLATION: Ground mount, SunLink Large-Scale Ground Mount System (GMS), 180° azimuth, 25° tilt

SOURCE CIRCUIT COMBINERS: 72 Bentek BTK26D-400A, 15 A fuses

ARRAY RECOMBINERS: Eight Bentek BTK-CBSS-ET-9400-S, 350 A and 400 A circuit breakers

SYSTEM MONITORING: AlsoEnergy DAS at each of four inverter pads, shared weather station

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